Caenorhabditis elegans has three POU homeobox genes, unc-86, ceh-6 and ceh-18. ceh-6 is the ortholog of vertebrate Brn1, Brn2, SCIP/Oct6 and Brn4 and fly Cf1a/drifter/ventral veinless. Comparison of C. elegans and C. briggsae CEH-6

Homeobox genes are a group of transcription factors that play important roles in development (for a review see Bürglin, 1995; Gehring et al., 1994). One particular class, the POU class of homeobox genes, was first defined by the transcription factors Pit1, Oct1, Oct2, and the Caenorhabditis elegans developmental control gene unc-86 (Herr et al., 1988). Since the initial identification of the POU homeobox gene class, more than 70 POU homeobox genes have been identified from a wide range of metazoans (Spaniol et al., 1996). They are expressed in patterns that vary from exquisitely cell type specific (for example, unc-86; Finney and Ruvkun, 1990; Finney et al., 1988) to ubiquitous (for example, Oct1; Sturm et al., 1988) and have been shown to regulate transcription of genes involved in a variety of processes (for a review see Ryan and Rosenfeld, 1997). Only three C. elegans POU homeobox genes can be identified in the virtually complete C. elegans genomic sequence (The C. elegans Sequencing Consortium, 1998): (1) unc-86 was identified genetically and shown to be necessary for the specification of particular neuroblasts and neurons (Finney et al., 1988; Finney and Ruvkun, 1990); (2) ceh-18, was identified by molecular probing and shown by reverse genetic analysis to play a distinct role in development, specifying ectodermal and gonadal cell fates (Greenstein et al., 1994; Rose et al., 1997); and (3) ceh-6, which was detected in degenerate oligonucleotide screens for C. elegans homeobox genes (Bürglin et al., 1989), but has not been functionally characterized. In this paper we examine the developmental role of ceh-6. Of the three C. elegans POU homeobox genes, ceh-6 can be assigned to the POU-III family, unc-86 to the POU-IV family (Bürglin et al., 1989; He et al., 1989) and ceh-18 to the POUII family (see below). We show that ceh-6 is the bona fide ortholog of the Drosophila gene Cf1a (Johnson and Hirsh, 1990) and of the four vertebrate genes Brn1, Brn2, SCIP (also known as Pou3f1, Tst1 or Oct6) and Brn4 (Hara et al., 1992; He et al., 1989; Mathis et al., 1992; Monuki et al., 1989). The fly gene Cf1a, shown to correspond to the genetic locus drifter (dfr), also known as ventral veinless (vvl), regulates a range of developmental processes, including tracheal formation (Anderson et al., 1995; de Celis et al., 1995). The vertebrate genes are expressed in many different areas of the nervous system, in particular in the brain and spinal cord (He 779 Development 128, 779-790 (2001) Printed in Great Britain © The Company of Biologists Limited 2001 DEV9713